3" Darkstar Stability

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michigander said:
I cried reading that build thread :( so sad
but would he add these days with Loki case out since then ? or wuss out ?

https://www.csrocketry.com/rocket-m...oki-research-m1378-loki-red-rocket-motor.html
Click to expand...

I have the 54\4000 case and have spoken to two others who have successfully flown it with the L2050 and I am not sure that a 3" Darkstar is the right rocket for that. However if you were really serious about it I would forgo the mid AV bay, use an extra long coupler to add some strength and screw it in place then go with HED.
 
I may chop mine too. First I need to sort out fixing the tailcone for EDIT gorilla motors when I installed it for aerotech. Silly me.
 
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DavidMcCann said:
I may chop mine too. First I need to sort out fixing the tailcone for Loki motors when I installed it for aerotech. Silly me.
Click to expand...
You just need a thin spacer. I used masking tape and built it up until it was high enough, maybe a dozen wraps. The width was only about an eighth of an inch.

Sent from my ONEPLUS A3000 using Rocketry Forum mobile app
 
mpitfield said:
You just need a thin spacer. I used masking tape and built it up until it was high enough, maybe a dozen wraps. The width was only about an eighth of an inch.

Sent from my ONEPLUS A3000 using Rocketry Forum mobile app
Click to expand...

I built it with an aerotech. When I put in Loki Gorilla cases I have a gap between the cone and the BT. Only solution I see so far is a tiny BT spacer, or to yank the aeropack, trim the MMT then reinstall.
 
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DavidMcCann said:
I built it with an aerotech. When I put in Loki or Gorilla cases I have a gap between the cone and the BT. Only solution I see so far is a tiny BT spacer, or to yank the aeropack, trim the MMT then reinstall.
Click to expand...

Didn't it come with a spacer? Every tail cone I bought, spacer was included.
 
timbucktoo said:
Didn't it come with a spacer? Every tail cone I bought, spacer was included.
Click to expand...


yep. just rechecked

Loki fits fine...it's the gorilla cases that gap.... maybe I'll leave it as is and just fly loki cases in it.
 
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DavidMcCann said:
I built it with an aerotech. When I put in Loki or Gorilla cases I have a gap between the cone and the BT. Only solution I see so far is a tiny BT spacer, or to yank the aeropack, trim the MMT then reinstall.
Click to expand...

I originally built mine with the GLR tailcone, but when I chopped and repainted it I swapped it for the Aeropack, same as yours Dave except black. I did this because the GLR tailcone was a bit tight with the Loki hardware and the internal snap ring was literally just hanging on. If you look in the pic below the ring is being flexed up. Anyway the Aeropack works fine, you just need a little spacer.



timbucktoo said:
Didn't it come with a spacer? Every tail cone I bought, spacer was included.
Click to expand...

The spacer that it comes with does not fit well with the Loki hardware, I can't recall if it is too big or small just that it was not a good fit.
 
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mpitfield said:
The spacer that it comes with does not fit well with the Loki hardware, I can't recall if it is too big or small just that it was not a good fit.
Click to expand...

That's odd. Loki is about all I use these days and have 4 rockets with tailcone and have no issues with fit.
 
Wasn't there a change in Loki hardware at some point where Scott moved the external thrust ring to a location that would match the dimension of the AT/ CTI thrust ring. I seem to remember something like that, but my memory could be faulty in this regard.
 
timbucktoo said:
That's odd. Loki is about all I use these days and have 4 rockets with tailcone and have no issues with fit.
Click to expand...

As I recall the spacer is too thick and as a result the retainer does not fully screw on.

markkoelsch said:
Wasn't there a change in Loki hardware at some point where Scott moved the external thrust ring to a location that would match the dimension of the AT/ CTI thrust ring. I seem to remember something like that, but my memory could be faulty in this regard.
Click to expand...

I am not sure, the 54-2800 case I flew in the Darkstar with the Aeropack is about three years old.
 
markg said:
I built a 3" Darkstar for my L2. I've flown it 3 times and all four times it's shown the same behaviour. It has a very pronounced 'wiggle' for the first 1000ft or so of flight and then it straightens out and flies perfectly. I've flown it on an Aerotech I500T, Aerotech J450DM, and a Loki K527. It's 10.1 pounds without a motor. The slowest it sims off the rail is on the J450DM at 45.5 ft/s, the I500 and K527 both sim at 58 ft/s. The wind varied on the flights, but it was not what I would consider excessive on any of them.

Here is a screenshot of OR. Stability is 6.15 cal with the I500 in it. Stability is 4.41 cal with the K527.

View attachment 327022

When I launched it at NYPower in May, I talked with another guy who had built a 3" Darkstar. He told me that he'd had the same wiggle on his and cured it by chopping about a foot off the overall length of the rocket and putting a lighter nosecone on it.

I ran simulations in OR to compare stability on the rocket flying on the J450 at it's normal 88" length and again with 12" removed. It shows the rocket is less stable with the 12" removed. Here are the two graphs :

Full
View attachment 327023

Cut
View attachment 327024

I'll attach the OR file as well.

Can someone help me understand why this is happening and if chopping 12" off the rocket will help or hinder this problem?

cheers - mark
Click to expand...

timbucktoo said:
Curious - how long is the rail you are using? On a Loki K527, mine is about 71 fps off the rail. Many flights and always straight as an arrow.
Click to expand...
Ditto. Mine flies straight as an arrow every time. I did my L2 on a CTI J400, as it was smooth as silk.
I'd be curious as to ground equipment. How steady are the launch pads? Sitting on firm ground? Otherwise, I don't know. Mine is fine. Good luck in solving the problem!
 
CzTeacherMan said:
Ditto. Mine flies straight as an arrow every time. I did my L2 on a CTI J400, as it was smooth as silk.
I'd be curious as to ground equipment. How steady are the launch pads? Sitting on firm ground? Otherwise, I don't know. Mine is fine. Good luck in solving the problem!
Click to expand...

ground equipment isn't going to cause this.
 
CzTeacherMan said:
Ah, I see.
I disagree that the issue at hand is actually coning.
Click to expand...

a fair stance. However, I've seen my photos, and others videos, and it certainly seems like it is. I'll dig out my photos and see if it shows it well, but it's a spiral smoke trail at 5-7,000 feet that's too smooth to be the smoke drifting
 
DavidMcCann said:
a fair stance. However, I've seen my photos, and others videos, and it certainly seems like it is. I'll dig out my photos and see if it shows it well, but it's a spiral smoke trail at 5-7,000 feet that's too smooth to be the smoke drifting
Click to expand...
Ah. That's possible. I was referring to the original post that said it straightened out after 1000 feet. Definitely a different story at 5k
 
CzTeacherMan said:
Ah. That's possible. I was referring to the original post that said it straightened out after 1000 feet. Definitely a different story at 5k
Click to expand...

Could have taken a gust and then worked its way out of it. It's a wacky thing. And to be fair, ground equipment -could- initiate it, but the cause is small fins, over stable, and some rotation. Perfectly aligned fins might avoid it, but would still have a swaying if knocking off an arrow straight flight... dunno.
 
Apologies to the OP for the thread hijack, however after thinking about the different Loki experiences with the Aeropack Tailcone, I figured there has to be something to it.

So I checked out my Loki hardware. Fortunately I have the full Loki 54mm set including two 54-2800 cases. One is older and one is newer with a lighter blue anodizing.



If I remove the external snap-ring from one of the cases and line it up with the other, by inserting the outside edge of the snap-ring into the other cases snap-ring groove, there is a visible difference.



That difference is roughly .05” closer to the aft end on the new case as compared to the old case.

Also overall the new case overall length is shorter. I did not measure it but with the snap-ring grooves still lined up you can see that the lighter anodized case is also shorter on the forward end.



The difference between where the snap-ring groove lines up does not sound like much, but when I use the newer case in my Darkstar, with the Aeropack spacer on the forward side of the snap ring, it fits perfectly…almost.



Using the spacer on the aft side of the snap-ring using the new and old case results in roughly 3/16ish recess which is sloppy play.

36815060782_773d3db47a.jpg
36589138040_53c8f8caf8.jpg


With the spacer on the forward side of the snap ring on my older set of hardware it results in the issue I mentioned in a previous post where the retainer does not fully seat.

36815060582_f649b39b23.jpg


With the newer hardware the retainer fully seats with the spacer on the forward side of the snap-ring and with a very marginal amount of play, certainly not enough to be a concern in my opinion.

So Tim’s AKA timbucktwo, experience is completely valid, as is mine. It needs to be qualified based on the hardware. Now the next question I would have is are the differences based on generational differences in hardware, or tolerances in the manufacturing process.
 
I guess Scott can possibly answer the "generational" difference. I've got 2 54mm cases that are the darker emerald color and I put the spacer on the foreword side of thrust ring for perfect fit.
I thought the different color in cases was due to the different anodizing, type II vs. type III (hardcoat). Guess there's more to it. Dunno.
 
mpitfield said:
Apologies to the OP for the thread hijack, however after thinking about the different Loki experiences with the Aeropack Tailcone, I figured there has to be something to it.

So I checked out my Loki hardware. Fortunately I have the full Loki 54mm set including two 54-2800 cases. One is older and one is newer with a lighter blue anodizing.



If I remove the external snap-ring from one of the cases and line it up with the other, by inserting the outside edge of the snap-ring into the other cases snap-ring groove, there is a visible difference.



That difference is roughly .05” closer to the aft end on the new case as compared to the old case.

Also overall the new case overall length is shorter. I did not measure it but with the snap-ring grooves still lined up you can see that the lighter anodized case is also shorter on the forward end.



The difference between where the snap-ring groove lines up does not sound like much, but when I use the newer case in my Darkstar, with the Aeropack spacer on the forward side of the snap ring, it fits perfectly…almost.



Using the spacer on the aft side of the snap-ring using the new and old case results in roughly 3/16ish recess which is sloppy play.

36815060782_773d3db47a.jpg
36589138040_53c8f8caf8.jpg


With the spacer on the forward side of the snap ring on my older set of hardware it results in the issue I mentioned in a previous post where the retainer does not fully seat.

36815060582_f649b39b23.jpg


With the newer hardware the retainer fully seats with the spacer on the forward side of the snap-ring and with a very marginal amount of play, certainly not enough to be a concern in my opinion.

So Tim’s AKA timbucktwo, experience is completely valid, as is mine. It needs to be qualified based on the hardware. Now the next question I would have is are the differences based on generational differences in hardware, or tolerances in the manufacturing process.
Click to expand...

As I mentioned earlier, Scott made the change so his newer motors would fit Aeropack. I believe this happened after he purchased Loki.
 
markkoelsch said:
As I mentioned earlier, Scott made the change so his newer motors would fit Aeropack. I believe this happened after he purchased Loki.
Click to expand...

Thanks Mark, I just received confirmation from Scott on this. Below is a cut and paste from his reply. Bottom line anyone considering using the new Loki 54/2800 and 54/4000 hardware can feel confident that it will fit in both the GLR Slimline retainer and the Aeropack Tail-cone retainer.

Reply from Scott:

"The difference in length between the two cases is basically taking .050" off both ends and keeping the internal and external grooves in the same locations. That's why the newer case fits the retainer and why I made the change to the 54/2800 and 4000 cases. I did not realize that would allow them to fit the tail cone retainers from Aeropack. Fitting Slimline was the main thing I was after."
 

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